Extraterrestrial intelligence (often abbreviated ETI) refers to hypothetical intelligent extraterrestrial life. The question of whether other inhabited worlds might exist has been debated since ancient times. The modern form of the concept emerged when the Copernican Revolution demonstrated that the Earth was a planet revolving around the Sun, and other planets were conversely, other worlds. The question of whether other inhabited planets or moons exist was a natural consequence of this new understanding. It has become one of the most speculative questions in science and is a central theme of science fiction and popular culture.
In June 2020, astronomers from the University of Nottingham reported the possible existence of over 30 "active communicating intelligent civilizations", or Communicating Extra-Terrestrial Intelligent (CETI) civilizations (none within our current ability to detect due to various reasons including distance or size) in our own Milky Way galaxy, based on the latest astrophysical information.
Intelligence is along with the more precise concept of sapience used to describe extraterrestrial life with similar abilities as humans. Another interchangeable term is sophoncy first coined by Karen Anderson and published in the 1966 works by her husband Poul Anderson.
Sentience, like consciousness are concepts sometimes mistakenly used for the concept of extraterrestrial sapience and intelligence, since they insufficiently describe such life.
The term of extraterrestrial civilization frames a more particular case of extraterrestrial intelligence. Its the possible longterm result of intelligent and specifically sapient extraterrestrial life.
The Copernican principle is generalized to the relativistic concept that humans are not privileged observers of the universe. Many prominent scientists, including Stephen Hawking have proposed that the sheer scale of the universe makes it improbable for intelligent life not to have emerged elsewhere. However, Fermi's Paradox highlights the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilization and humanity's lack of contact with, or evidence for, such civilizations.
The Kardashev scale is a speculative method of measuring a civilization's level of technological advancement, based on the amount of energy a civilization is able to utilize.
The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.
There has been a search for signals from extraterrestrial intelligence for several decades, with no solid results. Active SETI (Active Search for Extra-Terrestrial Intelligence) is the attempt to send messages to intelligent extraterrestrial life. Active SETI messages are usually sent in the form of radio signals. Physical messages like that of the Pioneer plaque may also be considered an active SETI message.
Communication with extraterrestrial intelligence (CETI) is a branch of the search for extraterrestrial intelligence that focuses on composing and deciphering messages that could theoretically be understood by another technological civilization. The best-known CETI experiment was the 1974 Arecibo message composed by Frank Drake and Carl Sagan. There are multiple independent organizations and individuals engaged in CETI research.
The U.S. government's position is that "chances of contact with an extraterrestrial intelligence are extremely small, given the distances involved." This line of thinking has led some to conclude that first contact will most likely be made with extraterrestrial artificial intelligence, rather than with biological beings.
The Wow! signal remains the best candidate for an extraterrestrial radio signal ever detected.
The potential changes from extraterrestrial contact could vary greatly in magnitude and type, based on the extraterrestrial civilization's level of technological advancement, degree of benevolence or malevolence, and level of mutual comprehension between itself and humanity. Some theories suggest that an extraterrestrial civilization could be advanced enough to dispense with biology, living instead inside of advanced computers. The medium through which humanity is contacted, be it electromagnetic radiation, direct physical interaction, extraterrestrial artefact, or otherwise, may also influence the results of contact. Incorporating these factors, various systems have been created to assess the implications of extraterrestrial contact.
The implications of extraterrestrial contact, particularly with a technologically superior civilization, have often been likened to the meeting of two vastly different human cultures on Earth, an historical precedent being the Columbian Exchange. Such meetings have generally led to the destruction of the civilization receiving contact (as opposed to the "contactor", which initiates contact), and therefore destruction of human civilization is a possible outcome. However, the absence of any such contact to date means such conjecture is largely speculative.
The extraterrestrial hypothesis is the idea that some UFOs are vehicles containing or sent by extraterrestrial beings (usually called aliens in this context). As an explanation for UFOs, ETI is sometimes contrasted with EDI (extradimensional intelligence), for example by Allen Hynek.
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